This invention was conceived in relation to the control problem associated with the operation of a fixed pitch prime mover, especially a wind turbine for example, in relation to applications where the turbine is required to operate in a "stand alone" mode providing power of substantially constant voltage and frequency for one or several consumers. The invention has no less merit in the application of variable pitch turbines to similar stand alone applications, in particular for example, where the wind energy has to be distributed to a number of specific locations.
The basic relationship is shown in FIG. 1 between the available energy in the moving air stream and the velocity of the air stream is displayed in curve A (broken line) where the power follows a cube law p=kV.sup.3. The performance curve of a typical constant speed fixed pitch turbine is shown by the full line curve B. There is no output from the turbine until the wind speed has reached that value at which it can drive the turbine at the correct speed to produce the required frequency. When the wind speed rises further the output of the turbine will rise approximately linearly until the aerofoil of the turbine blades is operating at its optimum angle of attack to the relative air stream for most efficient operation. The efficiency of the turbine is now at its maximum as shown by the chain dotted curve C. A further increase in the wind speed will cause the air turbine wing to become progressively stalled as the angle of attack is increased. The efficiency of the air turbine begins to fall and the output power of the turbine tends to a limiting value before it also begins to decrease as the wind speed increases yet further.
If now the turbine is allowed to run at a higher RPM the turbine efficiency curve is moved to the right as shown by dotted curve D, and the available energy in curve A is considerably increased at the wind speed at which maximum turbine efficiency is obtained resulting in a significant increase in the power available from the turbine and in the limiting power as the wing goes into stall.
Since the mechanical components of the aerogenerator such as the turbine shaft, the gearbox and the generator are rated for a specific power level corresponding to the maximum expected power it is important that the turbine speed is controlled in order to limit the maximum power which may be passed through the transmission system and the generator.
The maximum power which can be produced by the turbine is proportional to the RPM.sup.3 thus 5% increase in turbine speed will give an extra 15% power and conversely a reduction of 5% in the turbine speed will reduce the power by 15%.
This explanation has been included in order that the reason for an accurate and stable speed control system for the operation of a stand alone turbine may be fully understood in relation to the protection of the machinery and the optimum operating efficiency of the plant. This requirement is paramount and overrides the more commonly accepted reason for constant speed in that it relates to a constant frequency output.
It is necessary to distinguish between the need for limiting the maximum speed of the turbine as a power level limit and the operation of the turbine at slightly reduced speeds in low wind conditions where the improvement in the turbine efficiency at lower speed will give an enhanced output.
The energy in the wind is by definition an uncontrollable quantity owing to the variable nature of the wind speed from moment to moment. The control problem is analogous to that of a car running down a hill of variable slope and the speed there can be controlled by applying a variable braking effort as the slope varies. In the case of an aerogenerator this may be done automatically by providing a load characteristic similar to that depicted in FIG. 2, where no load is applied to the machine until the speed of the machine has risen to the required operating speed No. As the speed rises above this value the load is increased rapidly until the maximum power is reached P max. at a slightly higher speed Nm. A slight positive increase in speed is necessary in order that a stable control system may be constructed.
A known method whereby this control may be effected is shown in FIG. 3. Here the speed of the air turbine is monitored by means of the tachometer T which may be mounted on the turbine shaft or the generator shaft. The output from the tachogenerator is compared to a reference and when the tachogenerator output increases above the reference value, a signal is passed to a regulator control system which progressively turns on a power regulator allowing the energy in the busbars to be dissipated to a resistive dump load. Typically the power regulator could by a thyristor stack arranged to change progressively from fully off to fully on over the required speed range.
The deficiencies of this form of control are summarised briefly as follows:
The long term stability of the tachogenerator may cause a drift in the operating speed up or down and lead to an overload condition, or alternatively a loss of output.
Likewise a drift on the reference will have a similar effect. The system is vulnerable to loss of tacho signal or reference power supply.
Also a fault in the regulator control, regulator itself, or the unit dump load could lead to a loss of control or a serious unbalance in the case of a multi-phase alternator and busbar installation.
The use of a thyristor regulator in conjunction with a generating source of finite impedance will produce considerable harmonics on the busbars.
The resistive dump load must be sited within a reasonable distance of the air turbine in order to avoid unnecessary length of signal wires between the tachogenerator and the regulator control with the consequent risk of damage and vulnerability to pick up and interference.